tectonics Flashcards
how are earthquakes distributed
Most earthquake zones are found at, or close to, tectonic plate boundaries, often in clusters. About 70% of all earthquakes are found in the ‘Ring of Fire’ in the Pacific Ocean.
what are the most powerful earthquakes associated with
The most powerful earthquakes are associated with convergent or conservative boundaries.
what is the oceanic fracture zone (OFZ)
a belt of activity through the oceans along the mid-ocean ridges, coming ashore in Africa, the Red Sea, the Dead Sea rift and California.
what is the continental fracture zone (CFZ)
a belt of activity following the mountain ranges from Spain via the Alps, to the Middle East, the Himalayas to the East Indies and then circumscribing the Pacific.
how do earthquakes occur
Earthquakes are a sudden release of stored energy. As two plates move past each other they inevitably ‘stick’. This allows strain to build up over time and the plates are placed under increasing stress. Earthquakes are generated because of the sudden release of the stress
A pulse of energy radiates out in all directions from the earthquake focus. In some cases the earthquake motion displaces the surface, so a fault scarp can be seen.
how are volcanos distributed
they occur at, or close to tectonic plate boundaries, with the exception of hotspots, such as that by Hawaii.
what percentage of tsunamis occur in the pacific basin
90% of all events occurring in the Pacific Basin
This is due to the activity at the plate margins
how are tsunamis distributed
The global distribution of tsunami is fairly predictable in terms of source areas (90% Pacific basin)
Most are generated at subduction zones (convergent boundaries), particularly off the Japan-Taiwan island arc and South America
what are the causes of tsunamis
Tsunami are generated when a sub-marine earthquake displaces the sea bed vertically as a result of movement along a fault line at a subduction zone.
causes a bulge in water
The violent motion displaces a large volume of water, which then moves outwards in all directions from the point of displacement.
Sub-marine earthquakes that occur close to the shoreline can generate intense ground shaking damage, followed by the damage from the subsequent tsunami.
describe divergent (oceanic oceanic ) plate boundaries and what is found here
-Rising convection currents bring magma to the surface resulting in small, basaltic eruptions, creating new oceanic plate. (shield volcano- effusive eruptions VEI at 1-3)
-Minor, shallow earthquakes; low magnitude (5-6)
-can create an ocean ridge with central rift valley and volcanic islands.
describe divergent (continent continent) plate boundaries and what is found here
A geologically recent mantle plume splits a continental plate to create a new ocean basin. Mantle convection forces plates apart at constructive plate margins. Tensional forces open cracks and faults between the two planes (called a ‘fracture zone’) allowing for magma to move up and erupt
Basaltic volcanoes (shield)- effusive eruptions VEI at 1-3)
minor earthquakes- low magnitude (5-6)
describe convergent plate boundaries (oceanic oceanic) and what is found here
One oceanic plate is subducted beneath another, generating frequent earthquakes and a curving (arc) chain of volcanic islands, which erupt.
seismic activity: range of focal depths along Wadati benioff zone, moderate to high magnitude (7-9)
volcanic activity: explosive (moderate/high VEI scale 5-6)
describe convergent plate boundaries (continent continent) and what is found here
The collision of two continental landmasses, creating a mountain belt (fold mountains and plateaus) as the landmasses crumple, and magma is generated at depth. Infrequent major earthquakes distributed over a large area. Eruptions are possible, but as the magma cools and solidifies beneath the surface they are rare.
seismic activity: shallow to intermediate focal depth, moderate magnitude (6-8)
usually no volcanic activity
e.g. the Himalayas
describe convergent plate boundaries (continent oceanic) and what is found here
An oceanic plate is subducted under a continental plate, creating a volcanic mountain range, frequent large earthquakes and violent eruptions.
1.Mantle convection pulls plates towards subduction zones, whilst the other end of the plate is being pulled away at a divergent margin.
- Cold, dense oceanic plate is subducted beneath less dense continental plate; the density of the oceanic plate pulls itself into the mantle.
- The descending plate begins to melt at depth by a process called wet partial melting. This generates magma with a high gas and silica content, which erupts with explosive force.
-range of focal depths along the wadati benioff zone- often high magnitude (8-9)
-explosive, moderate to high VEI scale (5-6)
describe conservative (oceanic continent) and what is found here
Plates slide past each other, along zones known as transform faults.
Frequent earthquakes, with a shallow focal depth, so they can be very destructive if they are high magnitude. mag of 6-8
No volcanic activity.
ridges and scars on surface
E.g. San Andreas Fault, extends 1,200 km across California
what are intra-plate eruptions
intra plate eruptions are eruptions that take place away from a plate boundary at locations called mid-plate hotspots
-examples of these areas are Hawaii and the Galapagos islands
what are mantle plumes
mantle plumes are concentrated areas of heat convection. At plate boundaries, they are sheet-like, whereas at hot spots they are column like
at what speed to the earth plates move at per year
per year, the earths tectonic plates move at a speed of 2-5 cm per year
how thick is each tectonic plate
each plate is about 100km thick ( with a thinner oceanic/ continental crust on top)
why is the idea of plate tectonics still a theory
It’s still a theory because scientists have not yet directly observed the interior of the Earth.
what are key discoveries which confirm that plates move
- fossils confirm that places such Africa and South America have matching rock types. Fossils such as Cynognathus match across Africa and South America.
-In the 1950s, the ocean floor was surveyed showing the presence of mid ocean ridges where new crusts were formed.
-studies in the 1960s of magnetic strips in the oceanic crust confirm that tectonic plates move, these palaeomagnetic signals from past reversals of the Earth’s magnetic field and prove that new ocean crust is created by the process of sea-floor spreading at mid-ocean ridges. (seafloor spreading and palaeomagnetism occur at constructive margins, where new crust is being made) The striping due to palaeomagnetism revealed magnetic reversals in the Earth’s poles as oceanic crust moved away from the mid-oceanic ridges by seafloor spreading
-coal forms in tropical conditions however coal is found in the north sea
define subduction
the process of one plate sinking beneath another at a convergent plate boundary. Earthquakes at subduction zones occur at a range of focal depths from 10 km to 400 km, following the line of the subducting plate.
define slab pull
Cold, dense oceanic plate is subducted beneath less dense continental plate; the density of the the oceanic plate pulls itself into the mantle - this is slab pull. This occurs at destructive margins.
mantle convection: why is the interior of the Earth described as dynamic
Radioactive isotopes such as uranium-238 and thorium-232 in the Earth’s core and mantle generate huge amounts of heat which flow towards the Earth’s surface.
This heat flow generates convection currents in the plastic mantle.
All tectonic hazards are caused by the Earth’s internal heat engine.
what is the earths structure along with the states (g,l,s)
Crust
Mantle - a solid, but is deformable and capable of a very slow flow because of the very high temperatures.
Outer core - liquid
Inner core - solid
what are transform faults and give an example
transform faults are seen in conservative plate boundaries. They traverse the earths surface in a zig-zag pattern.
The most famous transform fault is the San Andreas fault which creates an area of frequent earthquake activity.
do earthquakes have shallow or deep focal depths and what does this mean for the earthquakes at conservative boundaries
Earthquakes along conservative boundaries often have shallow focal depths, meaning high magnitude earthquakes can be very destructive. Volcanic activity is absent.
what are all the characteristics of constructive margins
Eruptions are small and effusive, as the erupted basalt lava has a low gas content and high viscosity.
Earthquakes are shallow, less than 60 km deep, and have low magnitudes of under 5.0.
Oceanic-Oceanic: Minor, shallow earthquakes.
Continent-Continent: Basaltic volcanoes and minor earthquake
what are all the characteristics of destructive margins
Earthquakes at subduction zones occur at a range of focal depths from 10 km to 400 km, following the line of the subducting plate. This is called the Benioff Zone, and it can yield very large earthquakes up to magnitude 9.0.
The descending plate begins to melt at depth by a process called wet partial melting. This generates magma with a high gas and silica content, which erupts with explosive force.
Oceanic-Oceanic: Frequent earthquakes, and violent eruptions from curving chain of volcanic islands.
Continent-Continent: Infrequent major earthquakes distributed over a wide area.
Oceanic-Continent: Frequent large earthquakes and violent eruptions.
what are the characteristics of conservative margins
Earthquakes along conservative boundaries often have shallow focal depths, meaning high magnitude earthquakes can be very destructive. Volcanic activity is absent.
Oceanic-Continent: Frequent, shallow earthquakes, but no volcanic activity.
what is the focus of an earthquake
the focus is the point at which a pulse of energy radiates out in all directions from the earthquake focus
what is the epicentre of an earthquake
The epicentre is the point on the Earth’s surface directly above the focus.
what is a fault scarp
it is a feature resultant from an earthquake and it displaces the surface
what are the 3 seismic waves that earthquakes generate
-Primary waves
-Secondary waves
-Love waves
The overall severity of an earthquake is linked to the amplitude and frequency of these wave types.
give the features of P waves including:
-speed
-how damaging
-extra info
The fastest, (rate of about 8 km/sec) so they arrive first
Cause the least damage
They are vibrations causes by compression
give the features of S waves including:
-speed
-how damaging
-extra info
Arrive next (about 4 km / sec)
Shake the ground violently, causing damage
give the features of L waves including:
-speed
-how damaging
-extra info
Arrive last, as they travel only on the surface- ‘vibration occurring in the horizontal plane’
They have a large amplitude and cause significant damage, including fracturing the ground surface.
what does magnitude measure
magnitude measures the amount of energy released at the epicentre
how is earthquake magnitude measured
earthquake magnitude is measured using the moment magnitude scale (MMS)
MMS measures the energy released during an earthquake, it uses a logarithmic scale
what does an earthquakes intensity measure
an earthquakes intensity is the effect on people, structures and the natural environment
how is earthquake intensity meaured
by using the modified marcalli intensity scale which takes observations from people who experienced the earthquake
what are the primary effects (direct results) of an earthquake
ground shaking- causes buildings, bridges, roads and infrastructure to collapse, killing or injuring those nearby
crustal fracturing- when energy is released during an earthquake causes the Earths crust to crack -leaving gaps
what are the secondary effects of an earthquake
liquefaction- the violent shaking during an earthquake causes surface rocks to lose strength and become more liquid than solid. The subsoil loses its ability to support buildings. It can make rescue efforts more difficult and also disrupt underground power and gas lines.
landslides and avalanches- The ground shaking places stress on slopes, so they may fail (landslides, rockslides, mudslides and avalanches). This can lead to damage and injuries
tsunami- some underwater earthquakes generate tsunami that cause major problems for costal areas.
what are landslides
landslides are where material loses cohesive strength and moves downwards under the influence of gravity- they occur when seismic waves loose shock
give examples of where landslides have caused devastation
El Salvador, Central America
case study: What happened in Christchurch?
In 2011, a 6.5Mw earthquake occurred which had a shallow focus and the shockwaves were amplified by nearby solid rock. An upper soft layer slapped back down on the layer underneath sending renewed vibrations back to the surface, amplifying liquefaction
what impact did liquefaction have during the 2011 earthquake in Christchurch and why
areas built on diner deposits with a high water table were prone to damage due to liquefaction which causes loosely packed sediments to lose their strength.
-buildings lost their foundations
-subsidence movement downslope occurred
what happened in the urbanised region of Christchurch
-Fitzgerald Avenue (one of the main roads) was damaged due to 1.5m of lateral movement.
-Historic buildings such as the clocktower and College Hall needed reinforcing
how will structures in Christchurch be built in the future to adapt to future earthquakes
-some buildings such as the historic buildings (clock tower and college hall) have steel reinforced concrete walls within existing wall cavities, bracing floors and roofs with plywood diaphragms.
-steel cables threaded through high grade timber walls and frames then tightened.
-steel will absorb the energy from an earthquake
-incorporating concrete into gable ends, steel bars into stone columns (structural integrity)
In the Indian ocean, there was a tsunami in 2004, what was the wave height and how many people died
-the wave height was 24 metres
-230,000 deaths were present
what are tsunamis generated by
Tsunami can be generated by landslides or even the eruptions of volcanic islands, but most are generated by sub-marine earthquakes at subduction zones.
Tsunami are generated when a sub-marine earthquake displaces the sea bed vertically as a result of movement along a fault line at a subduction zone. The violent motion displaces a large volume of water in the ocean water column, which then moves outwards from the point of displacement.
The water then moves as a vast ‘bulge’ in open water
what were the economic impacts of the 2004 Indian ocean tsunami
-much infrastructure was destroyed
-economies were devastated, especially fishing, tourism and agriculture
-In Thailand, the tourism industry lost about US $25 million a month, and 120,000 workers lost their jobs.
what were the social impacts of the 2004 Indian ocean tsunami
-In Sumatra, 700,000 were left homeless.
-90 minutes in, it hits Thailand. 1700 tourists (many studying in Phuket) are included in the death toll.
-Communities destroyed so may not be supported enough by the loss in population.
-people displaced
-environmental refugees
what were the environmental impacts of the 2004 Indian ocean tsunami
-Ecosystems (such as mangroves, coral reefs, forests and costal wetlands were severely damaged.
-Freshwater supplies and agricultural soil were contaminated by salt water
-loss in biodiversity
explain the formation of a tsunami (3)
-tsunamis are created by water column displacement, mostly by undersea plate movements where part of the seabed is thrust upwards (or downwards) very quickly, or by explosive volcanic eruptions on a volcanic island, or underwater landslides such as continental shelf edges.
Factors making up a tsunami are: intensity, sequence, run-up and open ocean.
A tsunami consist of a sequence of waves with deep troughs in between because water is drawn upwards into each wave.
why was the 2004 tsunami (Indian ocean) so destructive
-It was caused by an earthquake with a magnitude between 9.0 and 9.3. Its thrust heaved the floor of the Indian ocean towards Indonesia by about 15 metres thus sending out shock waves
-the epicentre was close to some densely populated communities
- the tsunami could travel several kilometres inland due to the low lying nature of Indian Ocean countries.
-Affected Indian ocean countries
-There was no early warnings system in place and they did not ahve the resources (lower income)
-Mangrove forests prior to the tsunami were destroyed
what systems are used to detect early warning systems of tsunamis
systems using seismic sensors are used to detect undersea earthquakes
-DART systems are also used
whats the DART system
the DART (Deep-ocean Assessment and Reporting of Tsunami) uses seabed sensors and surface buoys to monitor changes in sea level and pressure.
when tsunami waves are detected, the system sends the information via a satellite to tsunami warning stations, these stations use computer modelling to estimate the size and direction of the tsunami
Japan has the most extensive earthquake and tsunami warning systems in the world.
whats an example of a super volcano
Yellowstone, USA
what is the shape of a volcano related to
the shape of the volcano is related to the type of lava erupted in which there are three main types: basalt, andesite and rhyolite
what are volcanoes
volcanoes are extrusive features found on the Earths surface, ranging from gentle fissure eruptions to explosive composite cones
define low viscosity
runny
define viscous
sticky
what are the features of basaltic lava
-hottest
-low silica, high CO2
-low gas content
-formed by melting of mantle materials
-thin and runny (low viscosity)
-gentle and effusive eruption energy
-location: ocean hot spots, mid-ocean ridges, shield volcanoes
what are the features of andesitic lava
- 800-1000°C
-medium silica and gas content
-formed when subducted oceanic plate melts and mixes with seawater, lithospheric mantle and continental rocks.
-flow is slow (intermediate viscosity)
-eruption energy is violent and moderately explosive
-located at composite cone volcanoes and subduction zones
what are the features of rhyolitic lava
-coolest (650 to 800°C)
-high silica (70%) and gas content.
-formed by melting of lithospheric mantle and slabs of previously subducted plate.
-highly viscous flow
-eruption energy is very violent
-located at super volcanoes and composite cone volcanoes
what type of lava does a shield volcano have
shield= basaltic (less viscous)
what type of lava does a composite cone volcano have
composite cone= andesitic lava (viscous)
explain the key differences between composite cone and shield volcanoes
composite cone volcanoes= form at convergent plate boundaries
shield volcanoes= form on divergent plate boundaries
composite cone volcanoes are more dangerous than shield volcanoes
what are the primary and secondary hazards that lava flows form
primary hazards:
-lava flow
-proclastic flow
-ash fall
-gas eruption
secondary hazards:
-lahars
-jokulhlaups
describe the volcanic hazard: lava flow
-lava is molten magma that has reached the earths surface
-basaltic lavas are less viscous and flow fastest so can extend over several kilometres
-occurs at subduction zone volcanoes (composite)
-occurs at a hot-spot volcano (shield type)
give an example of the effects of lava as a volcanic hazard
a flank fissure on the nyiragongo volcano in 1977 caused a lava lake to drain in less than one hour from the crater. It unofficially killed 400 people
describe the volcanic hazard: pyroclastic flow
very large, dense clouds of hot ash and gas at temperatures of up to 600’C.
pyroclastic flows are a dense mixture of superheated tephra’s (all types of ejected material) and poisonous gasses, moving rapidly down the sides of a composite volcano.
They can flow down the flanks of volcanoes and devastate large areas. Anything in the path of a pyroclastic flow is likely to be destroyed by impact, burying, burning or poison
Occurs at: Subduction zone volcano (composite)
describe the volcanic hazard: ash fall
ash particles, and larger tephra particles, can blanket large areas in ash, killing vegetation, collapsing buildings and poisoning water sources.
-occurs at constructive plate margins (fissure eruption) and subduction zone volcanoes (composite)
describe the volcanic hazard: gas eruption
The eruption of carbon dioxide and sulphur dioxide, which can poison people and animals in extreme cases.
occurs at: subduction zone volcanoes (composite) and hot-spot volcanoes (shield)
give an example of the effects of pyroclastic flows as a volcanic hazard
Chances Peak on Montserrat in 1997 destroyed the capital town of Plymouth with a pyroclastic flow
describe the volcanic hazard: Lahars
Volcanic mudflows, which occur when rainfall mobilises volcanic ash. They travel at high speed down river systems and cause major destruction.
they are fast and can travel great distances
they occur at subduction zone volcanoes (composite)
give an example of the effects of lahars as a volcanic hazard
The town of Armero, Colombia suffered from a lahar which killed 23,000 people
describe the volcanic hazard: Jokulhlaups
a jokulhlaup is a flood of meltwater issuing from underneath an ice cap or glacier. This happens when a volcano erupts underneath the ice and it melts the ice so that it forms a lake. Devastating floods can occur and they are common in Iceland.
occurs at constructive plate margin volcanoes- fissure eruption
give an example of the effects of jokulhlaups as a volcanic hazard
Occur in Iceland.
The main road along the south coast has often been cut and bridges washed away.
E15 volcano responsible for ash clouds and created Jokulhlaups
describe the volcanic hazard: tephra
tephra’s is all types of ejected material- an explosive volcano will eject solid and molten rock fragments into the air (tephra). Smaller fragments can move into the atmosphere , this can disrupt planes
give an example of the effects of tephra’s as a volcanic hazard
The 2010 eruption of EFJ in Iceland disrupted flight plans (disrupted flight plans to and from Europe for a week)
where are most tectonic hazards located
where there are large composite cone volcanoes at a destructive plate margin.
These eruptions often have lava flows, pyroclastic flows, lahars and extensive ash and tephra fall that can affect areas up to 30 km from the volcanic vent.
define natural hazard
A natural hazard is a natural event that has the potential to harm people and their property.
what is a disaster
-A disaster is the realisation of the hazard, i.e. harm has occurred.
-UN defines it as ‘a serious disruption of the functioning of a community or a society involving widespread human, material, economic or environmental losses and impacts, which exceeds the ability of the affected community or society to cope using its own resources.
how can we measure a disaster
-quantitively- e.g. number of deaths and cost of damage
OR
-Qualitatively- amount of upset within a community. However it is less difficult to measure the grief of those affected by disasters
what are mega disasters
mega disasters are when the GDP of a country is reduced by at least 5% OR over 2000 deaths OR over 20,000 people made homeless OR over a year of foreign aid dependency.
why are volcanoes generally less hazardous than earthquakes
Few volcanic hazards ever reach mega-disaster proportions due to their greater temporal and spatial predictability and smaller affected areas
why is the MMS (Moment Magnitude Scale) more accurate than the Richter scale
The MMS scale uses the energy released by all shockwaves as well as the area of rupture and movement whereas the Richter scale uses the arrival times of S and P waves, the amplitude of the S waves and the distance from the epicentre
what is magnitude
the energy released from an earthquake
what is intensity
the damage caused by an earthquake*
How do we measure the intensity/ damage caused by an earthquake
use the Mercalli scale
what is the VEI (intensity of a volcano)*
A logarithmic scale between 0 and 8 which is used to describe and compare the size or magnitude of volcanic eruptions.
what is the Deggs model
the Deggs model shows that disasters have to involve people and they occur at the intersection of people and hazards shown by the Degg Model
The Deggs model is a combination of physical factors (e.g. magnitude) and human factors (e.g vulnerable population/ lack of knowledge) that combine ro create a level of risk
A hazard (volcano or earthquake) + vulnerable population is a disaster
what is risk
risk is the probability of harm or loss taking place. This includes deaths, injuries, trauma and upset, loss of livelihoods, damage or loss of property and disruption to economic activities.
what is the PAR model (pressure and release model)
The par model suggests what should be tackled in order to reduce the risk of a disaster, such as root causes, dynamic pressures and unsafe living conditions.
The physical factors and processes are difficult to change, certainly earthquakes are impossible to predict. However, small things such as building barriers to divert lava flows may be possible as used on Mount Etna
what factors cause a high level of vulnerability
-weak governance and dysfunctional cultural, political and economic systems
-lack of local organisations, training, skills development
-unsafe living conditions (lack of resistant infrastructure)
what is the equation for risk
risk = (hazard x vulnerability)
/(capacity to cope)
what is risk
risk is the probability of harm or loss taking place. This includes deaths, injuries, trauma and upset, loss of livelihoods, damage or loss of property and disruption to economic activities
what is a hazard
a hazard is an earthquake or volcanic event itself
ways to reduce vulnerability:
-better strategies and organisation/ prepared
-better technology
-knowledge and understanding on how to react
summarise the differences between prediction between earthquakes and volcanoes
-for predicting earthquakes, estimation of build up of stress is issued, this is by using the time gaps between earthquakes
-Volcanoes can be predicted by harmonic tremors, gas and steam emissions or buldges in the crater side of the volcanoes. Around 50 volcanoes erupt each year
what does the PAR model look at
looks at the underlying causes of disaster
what were the root causes causing vulnerability for Haiti by the 2010 earthquake
-Haiti was heavily in debt
-corruption in government
-government budget came from foreign aid (30-40%)
what were the dynamic pressures causing vulnerability for Haiti by the 2010 earthquake
-lack of urban planning control
-lack of effective education
-rapid urbanisation occurred which formed unstable slum like houses
what were the unsafe conditions causing vulnerability for Haiti by the 2010 earthquake
-buildings constructed in soft soil
-illegal housing in unsafe areas
-low GDP per capita $1300
how did Haiti recover from the 2010 earthquake ( parks model)
-5 years after the earthquake it was still recovering
-lots of aid received as not self sufficient
-2015- 80,000 people still living in temporary housing
how many people died in the Hati 2010 earthquake
300,000 people died
emerging country china: what was the immediate impact of the 2008 earthquake
-5 million made homeless
-concentrated in rural areas (not as densely populated)
-chinas central gov was able to respond quickly instead of the corrupt gov officials
-gov pledged $10 billion for building works
emerging country: what was the long term impact of the 2008 earthquake
gov saw an opportunity to rebuild the area
-over 200 transport projects were under construction or had been completed ( increase QoL)
what was the root cause causing vulnerability due to the earthquake and tsunami in Japan 2011
-oversight by gov in building nuclear power plant
what was the dynamic pressure causing vulnerability due to the earthquake and tsunami in Japan 2011
nuclear powerplant not made to withstand the impact of tsunami
what was the unsafe conditions causing vulnerability due to the earthquake and tsunami in Japan 2011
-the Fukushima nuclear power plant was vulnerable and unsecure
what was the immediate response to the 2011 Japan earthquake and tsunami
-immediate gov response
-all radio stations told people what to do
-quickly accepted help from over 20 countries
what were the long term effects of the Japan 2011 earthquake and tsunami
-nuclear energy declined to 1% by 2013
-japan had to start importing fossil fuels
-greenhouse gas emission increase
-pushed energy prices up
